Fix issue with Ruby client where strings from example properties are not wrapped with quotes

Created by: ndbroadbent

This fixes an issue with examples for the Ruby client, where example strings are not properly quoted.

Before, it would generate code like this:

order_id = order_id123 # String | 

After this change, the example string is wrapped with single quotes:

order_id = 'order_id123' # String | 

This change might be useful for some other languages too, but not sure. Maybe I should update the code in DefaultCodegen.